Components in computer systems are operating at higher and higher frequencies, using smaller die sizes and more densely packed circuitry. As a result, these components, especially microprocessors, generate large amounts of heat, which must be removed from the system's chassis so that the components do not overheat. In conventional computer systems, this is accomplished via forced air convection, which transfers heat from the circuit components by using one or more fans that are disposed within or coupled to the chassis to draw air over the components through the chassis. To further aid the heat removal process, heat sinks are often mounted to various high-power circuit components to enhance natural and forced convection heat transfer processes. Heat sinks comprising of an array of fins having a height of approximately 1-2 inches are commonly used to cool microprocessors in desktop systems, workstations, and pedestal-mounted servers. The heat sinks provide significantly greater surface areas than the components upon which they are mounted.
For example, a typical processor cooling solution that employs a heatsink is shown in FIG. 1. The cooling solution is designed to cool a processor die 100, which is flip-bonded to a substrate 102 via a plurality of solder bumps 104. Typically, an epoxy underfill 106 is employed to strengthen the interface between die 100 and substrate 102. Substrate 102, in turn, is mounted to a chip carrier 108 via a plurality of solder balls 110. The upper side of the die is thermally coupled to a copper heat spreader 112 via a first layer of thermal interface material (TIM) 114. Similarly, a heat sink 118 is thermally coupled to the copper heat spreader via a second layer of TIM 118.
During operation, processor die generates heat due to resistive losses in its circuitry. This heats up the processor. Since heat flows high temperature sources to lower temperature sinks, heat is caused to flow through TIM layer 114 to copper spreader 112. In turn, heat from the spreader flows through TIM layer 118 to heat sink 116. The heat sink, in turn, is cooled by air that flows over the heat sink's fins 120, either via natural convection or forced convection. Generally, the rate of cooling is a function of the fin area and the velocity of the air convection.
Thermal solutions are even more difficult for smaller processor-based devices, such as laptop computers and the like. In this instance, the amount of space available for heat sinks and heat spreaders is minimal, thereby causing the heat transfer capacity to be significantly reduced. The power available to drive fans is also significantly reduced. Even with the use of lower-power dies, the reduced heat transfer capacity often leads to the processors running derated speeds via self-regulation in response to over temp conditions.